Protein kinases are families of enzymes that catalyze the phosphorylation of specific residues in proteins, broadly classified into tyrosine and serine/threonine kinases. Inappropriate kinase activity, arising from mutation, over-expression, or inappropriate regulation, dys-regulation or de-regulation, as well as over- or under-production of growth factors or cytokines has been implicated in many diseases, including but not limited too cancer, cardiovascular diseases, allergies, asthma and other respiratory diseases, autoimmune diseases, inflammatory diseases, bone diseases, metabolic disorders, and neurological and neurodegenerative disorders such as Alzheimer's disease. Inappropriate kinase activity triggers a variety of biological cellular responses relating to cell growth, cell differentiation, survival, apoptosis, mitogenesis, cell cycle control, and cell mobility implicated in the aforementioned and related diseases.
Protein kinases have emerged as an important class of enzymes as targets for therapeutic intervention. In particular, the JAK family of cellular protein tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2) play a central role in cytokine signaling (Kisseleva et al, Gene, 2002, 285, 1; Yamaoka et al. Genome Biology 2004, 5, 253)). Upon binding to their receptors, cytokines activate JAK which then phosphorylate the cytokine receptor, thereby creating docking sites for signaling molecules, notably, members of the signal transducer and activator of transcription (STAT) family that ultimately lead to gene expression. Numerous cytokines are known to activate the JAK family. These cytokines include, the IFN family (IFN-αs/β/ω/Limitin, IFN-γ, IL-10, IL-19, IL-20, IL-22), the gp130 family (IL-6, IL-11, OSM, LIF, CNTF, NNT-1/BSF-3, G-CSF, CT-1, Leptin, IL-12, IL-23), γC family (IL-2, IL-7, TSLP, IL-9, IL-15, IL-21, IL-4, IL-13), IL-3 family (IL-3, IL-5, GM-CSF), single chain family (EPO, GH, PRL, TPO), receptor tyrosine kinases (EGF, PDGF, CSF-1, HGF), and G-protein coupled receptors (AT1).
Until recently, the therapeutic potential of JAK inhibitors has focused on diseases affecting various pathologies of the immune system. These include, but are not limited to atopy (allergic asthma, atopic dermatitis, allergic rhinitis), cell mediated hypersensitivity (allergic contact dermatitis, hypersensitivity pneumonitis), rheumatic diseases (systemic lupus erythematosus (SLE), rheumatoid arthritis, juvenile arthritis, Sjogren's Syndrome, scleroderma, polymyositis, ankylosing spondylitis, psoriatic arthritis), transplantation (transplant rejection, graft vs host disease), viral diseases (Epstein Barr Virus, Hepatitis B, Hepatitis C, HIV, HTLV1, Vaicella-Zoster Virus, Human Papilloma Virus), cancer (leukemia, lymphoma), cardiovascular disease (cardiac hypertrophy, atherosclerosis and arteriosclerosis), neurodegenerative diseases (motor neuron disease), food allergy, inflammatory bowel disease, Crohn's disease, cutaneous inflammation, and immune suppression induced by solid tumors. Most efforts to date have targeted JAK3 inhibition for immunosuppression, for example organ transplantation and allograft acceptance (for a review, see Borie et al. Current Opinion in Investigational Drugs, 2003, 4(11), 1297).
Most recently, two significant findings of the role of the EPO-JAK2 signaling pathway in myeloproliferative disorders and proliferative diabetic retinopathy were found. First, a gain-of-function, somatic (acquired) mutation of the JAK2 kinase (V617F) was reported to be a causative factor in a number of “typical” myeloproliferative disorders, including polycethemia vera, essential thrombocythemia and melofibrosis with myeloid metaplasia, and the mutation has been found in patients with either “atypical” myeloproliferative disorders and myelodysplastic syndrome (for reviews see Tefferi and Gilliland, Cell Cycle 2005, 4(8), e61; Pesu et. al. Molecular Interventions 2005, 5(4), 211). Additionally it was found that (a) the V617F JAK2 mutation was associated with constitutive phosphorylation of JAK2 and its downstream effectors as well as induction of erythropoietin hypersensitivity in cell based experiments, (b) V617F JAK2-indcued cell proliferation signals were inhibited by small molecule inhibitors of JAK2, and (c) murine bone marrow transduced with a retrovirus containing V617F JAK2 incuded erythrocytosis in the transplanted mice.
Furthermore, recently it has been found that mutations in EPO-R also keep the JAK pathway constitutively activated leading to myleoproliferative disorders.
Second, EPO was found to be a potent angiogenic factor in proliferative diabetic retinopathy, a major cause of vision loss affecting diabetic, working-age persons (see for example Aiello, New England Journal of Medicine, 2005, 353 (8), 839; Watanabe et al. New England Journal of Medicine 2005 353 (8), 782).
Further, findings from the Watanabe research showed (a) intraocular EPO levels and VEGF (another well-known angiogenic factor in proliferative diabetic retinopathy) were significantly higher among those with proliferative diabetic retinopathy than those with quiescent disease or non-diabetic control, (b) EPO and VEGF levels were not closely correlated, (c) EPO levels were more strongly correlated with the presence of proliferative diabetic retinopathy than VEGF, (d) EPO stimulated growth and intracellular signaling in retinal endothelial cells, and (e) inhibitors of either EPO or VEGF reduced hypoxia-induced retinal neovascularization in rodent models.
Recently it has been shown that mutations in the EPO receptor may also affect the signaling related to the JAK pathway and this may have implications in terms of disease states where JAK signaling is important in the cell cycle.
There is another feature regarding inhibitors of the JAK pathway. It has been demonstrated that the JAK pathway may be recruited in cell survival and proliferation. For example, in the case of the cells that are Philadelphia chromosome positive that result in chronic myelogenous leukemia (CML), there is evidence that the Jak pathway is recruited in constitutive activation. Accordingly, using a JAK inhibitor may have use in CML in which the Philadephia chromosome has been shown to produce the hybrid Bcr-Abl, thus keeping cells constitutively active.
More telling is that in cases of resistance mutations that arise on account of specific inhibitors to BCR-ABL, as in the case of the T315I gatekeeper mutation, or any other mutation, it may be possible to use a JAK inhibitor on account of the pathway used by the BCR-ABL mutant (as in the case of BCR-ABL(T315I) mutation) utilizing the Jak pathway. Thus Jak inhibitors may be used in the treatment of patients with resistance to known therapies where BCR-ABL is directly targeted and drug resistance has now been shown as the dominant (50-90%) of all resistance in patients where existing therapies fail.
The use of JAK inhibitors may also find utility in other myeloid disease states, both blood disorders and other disease states with myeloid implications, and other disease states in which the JAK pathway is implicated directly or indirectly.
Accordingly, there is a need to develop compounds useful as inhibitors of kinases, particularly, JAK kinase, given the inadequate treatments available for the aforementioned diseases where the JAK signaling pathway is dysregulated, or recruited directly or indirectly.